Re: positive and negative work

[Jack Uretsky <jlu@HEP.ANL.GOV>]

I don't think anyone has answered Larry's question, and, heaven knows, I'm
no expert.  I have an understanding that the answer goes something like
this:
        The muscles such as biceps used in lifting heavy objects are
"activated" to alternately flex and relax (many fibers twitching
asynchronously).  The flexing of a muscle fiber is triggered by a nerve
impulse, but the actual flexing is a (mysterious to me) chemical process
in which energy is supplied by red blood cells - which are oxygen
suppliers - and lactic acid (sour milk) is an end product.  I suppose this
means that lactose is "burned" in the process.  This must mean (I
speculate) that a flexed muscle fiber is like a motor that is turning an
axle against a friction brake.  I  think that there is actual warming of the
region in the vicinity of the flexed muscle.
        The mechanics of the flexing process must be quite marvelous.

        This explanation is consistent with, but somewhat more detailed,
than Feynman's




On Fri, 26 Sep 2003, Larry Smith wrote:

> At 11:55 AM -0500 11/8/01, John S. Denker wrote:
> > I wrote:
> >
> > > The book is wrong.  The work done _by_ the muscles is negative.  (The work
> > > done _on_ the muscles would be positive, but that's not the question that
> > > was asked.)
> >
> > By the way:  If you look at the biophysics, it turns out that the negative
> > work
> > done by the muscles does not show up as useful mechanical or chemical
> > energy in
> > the muscles.  Indeed you need to provide a positive amount of chemical
> > energy to
> > allow the muscle to do its negative work.
> >
> > However, that does not change my answer.   Negative work is done by the
> > muscles
> > during sitting.
> >
> > (You could summarize this by saying that the muscle is more-than-100%
> > inefficient
> > in storing the energy.)
>
> A student asked to day "If I don't do any work holding a heavy object, why
> do I get tired after a while?"  Is John's answer something I should pass
> on?  What's the chemistry of getting tired?
>
> Thanks,
> Larry

--
"Don't push the river, it flows by itself"
                                Frederick Perls